Bandgap properties of modified square photonic crystal fibers

The bandgap properties of modified square photonic crystal fibers are investigated. It is shown numerically that the bandgaps of such fibers can be tailored by introducing an extra air hole into the center of each unit cell of the original square lattice. Square photonic crystal fibers with large air holes can be designed to be air-guiding by introducing extra air holes of about the same size. The air-guiding regions of square photonic bandgap fibers with extremely large air holes can also be greatly extended by the extra air holes.